This invention relates to the field of digital communications. More specifically, the present invention relates to rotationally invariant digital communications and to phase constellation configurations that promote rotationally invariant digital communications.
For efficient data communications, transmit power levels should be as low as possible, data rate should be as high as possible, and bit error rate should be as low as possible. In all communication systems, faster data rates and lower bit error rates are desirable. In wireless communication systems, lower power levels are particularly desirable because they permit the use of less expensive transmit amplifiers, achieve better spectral containment within an allocated frequency band and reduce battery drain in battery-powered devices. Any one or two of these three characteristics can often be improved at a cost of deteriorating the remaining characteristic(s) without an improvement in basic communication efficiency. However, an improvement in the basic communication efficiency is needed to improve one of these characteristics without causing deterioration in the others.
A phase point constellation defines the alphabet of phase states at which a digital communication signal is transmitted and assigns specific digital codes to those phase states. The phase point constellation configuration has a significant influence on basic communication efficiency. Older communication systems have used a QPSK phase constellation in which the alphabet includes only four phase points. Communication efficiency improves by using a more efficient phase point constellation. Thus, a phase point constellation with an alphabet of sixteen phase points is used in implementing the well-known and more efficient 16-QAM modulation. More modern communication systems use even higher order phase point constellations.
In a differentially coherent communication system, data is communicated through relative change in carrier phase between one unit baud interval and the next. In contrast, coherent communications communicate data through absolute carrier phase states in each unit baud interval. With differentially coherent communications, a receiver need not acquire or maintain the absolute phase reference used by the transmitter""s phase point constellation in forming the digital communication signal. Unfortunately, differentially coherent communications are less efficient and may experience up to 3 dB performance degradation when compared to an otherwise equivalent coherent communication scheme because errors that occur in one unit baud interval cause additional errors in the next unit baud interval. Consequentially, coherent communication systems are often preferred because of improved efficiency, but such systems must acquire an absolute phase reference before valid data may be detected.
Once a receiver in a coherent communication system acquires an absolute carrier phase reference, a cycle slip phenomenon may cause the receiver to lose that phase reference. The cycle slip phenomenon may result from the use of inexpensive, noisy oscillators in the transmitter and/or receiver, interference, and the like. When cycle slip happens, valid data can no longer be detected, and the receiver needs to reacquire the absolute carrier phase reference before valid data can again be detected.
A non-rotationally invariant communication system endures a lengthy absolute carrier phase reference acquisition and reacquisition process. Often, this lengthy process may be tens of thousands of unit baud intervals long. Partially rotationally invariant systems must also endure a lengthy acquisition or reacquisition process for some cycle slip situations, but can quickly resolve the proper carrier phase reference for other cycle slip situations. When the cycle slip phenomenon occurs more often than rarely, this reacquisition time may be undesirably long for non-rotationally invariant and partially rotationally invariant communication systems.
In contrast to non-rotationally invariant and partially rotationally invariant communication systems, fully rotationally invariant (FRI) systems can quickly resolve proper phase rotation (i.e. acquire the carrier phase reference) for all cycle slip situations within a few baud intervals. Unfortunately, conventional FRI communication systems are undesirably complicated, inefficient, or useable only with lower order modulations. For example, conventional FRI communication systems require the insertion of parity or pilot bits that are checked on frame boundaries in the receiver to resolve phase ambiguities. Such systems undesirably complicate the hardware by requiring time-framing circuits along with parity or pilot bit insertion, extraction and checking circuits. Such systems also experience inefficiency by dedicating communication capacity to overhead parity and pilot bits that could otherwise be applied to user data.
Other conventional FRI systems achieve full rotational invariance by adopting an excessively inefficient phase constellation. For example, conventional 16-QAM is not a rotationally invariant scheme, but can be altered into a rotationally invariant scheme by moving each quadrant""s phase points diagonally away from the phase point constellation origin. Unfortunately, this technique results in a communication scheme that can be even less efficient than differentially coherent 16-QAM communication, and the efficiency deteriorates as this technique is applied to larger modulation orders. Still other systems adopt phase constellations that lead to basic communication inefficiency when adapted for use with wireless communication links.
Still other FRI systems, while carrier phase coherent, rely on differential data encoding for all communicated bits. Differential data encoding, particularly when applied to all communicated bits, is undesirable for the same reason that differentially coherent carrier phase communications is undesirable. Namely, with differential data encoding, bit errors occur in pairs of unit intervals causing a deterioration in basic communication efficiency.
Accordingly, it is an advantage of the present invention that an improved rotationally invariant digital communication system and method are provided.
Another advantage is that an improved digital communication system and method achieve full rotational invariance even for higher modulation orders.
Another advantage is that rotational invariance is achieved at a cost of only a small decrease in basic communication efficiency.
Another advantage is that rotational invariance is achieved using a phase point constellation that is suitable for wireless communication.
Another advantage is that rotational invariance is achieved without requiring differential encoding on at least a portion of the communicated bits.
The above and other advantages of the present invention are carried out in one form by a data modulator used in a digital communications system. The data modulator includes phase map inputs for receiving at least four bits per unit baud interval. Phase map outputs provide a set of coordinates of a two-dimensional phase space during each unit baud interval. A phase mapping circuit is coupled between said inputs and said outputs. The phase mapping circuit is configured to implement a phase point constellation having an origin and having phase points collinearly arranged along four diameters of the phase point constellation. A first two of these four diameters intersect at substantially 90xc2x0, and a second two of these four diameters intersect at substantially 90xc2x0. The first two diameters are rotated relative to said second two diameters at substantially 45xc2x0, and the first two diameters have fewer phase points than the second two diameters.
The above and other advantages of the present invention are carried out in another form by a method for rotationally invariant communication in a digital communication system. The method calls for receiving a digital communication signal configured in accordance with a phase space having a constellation of at least sixteen phase points, each of which is addressed by at least two differentially encoded bits and two convolutionally encoded bits. For each of the phase points, there exists located at substantially the same magnitude as the phase point and rotated approximately 90xc2x0 in the phase constellation from the phase point, another phase point having an equal data value for the two convolutionally encoded bits. In response to the digital communication signal, phase error estimates relative to the constellation of at least sixteen phase points are generated. The phase space is masked into an enabled portion and a disabled portion. A phase locked loop is driven with phase error estimates obtained from the enabled portion of the phase space.